The Amiga's quite a unique design: in the standard config the CPU lives on the same bus as the chipset (gfx and such) where it has to share memory access and is thus slowed down at times. However, it is possible to detach the CPU (and the 'fast' memory dedicated to that side) from the chip bus and let it run on its own.
This is very nice when high load is put on the chipset bus but it even gets much better once you get the chance to run the CPU bus faster than the chipset bus - this is what an accelerator board does.
Every now and then the CPU will have to access RAM or registers on the chip bus, so it'll have to slow down to its speed, so that the memory cycles start and end at the same time. This is the required synchronisation I was referring to.
It is fairly easy (little extra logic required) to run the CPU at an exact multiple of the chip bus, but without cache there's not much use.
To make it short: there's no (simple) way you can upclock your CPU, whether it's an 8, 12 or 20 MHz type. A CPU with significant cache might get away with running on a slow bus, but the 68000 is not designed for that I'm afraid.
